Add result<U&, E> specialization. Fixes #72.

test/result_emplace.cpp

@@ -172,5 +172,47 @@ int main()
         BOOST_TEST_EQ( Y::instances, 0 );
     }
 
+    {
+        int x1 = 1;
+        result<int&> r( x1 );
+
+        BOOST_TEST( r.has_value() );
+        BOOST_TEST_EQ( r.value(), 1 );
+
+        int x2 = 2;
+        r.emplace( x2 );
+
+        BOOST_TEST( r.has_value() );
+        BOOST_TEST_EQ( r.value(), 2 );
+    }
+
+    {
+        result<int&> r( ENOENT, generic_category() );
+
+        BOOST_TEST( !r.has_value() );
+
+        int x2 = 2;
+        r.emplace( x2 );
+
+        BOOST_TEST( r.has_value() );
+        BOOST_TEST_EQ( r.value(), 2 );
+    }
+
+    BOOST_TEST_EQ( Y::instances, 0 );
+
+    {
+        result<int&, Y> r( in_place_error );
+
+        BOOST_TEST( !r.has_value() );
+        BOOST_TEST_EQ( Y::instances, 1 );
+
+        int x2 = 2;
+        r.emplace( x2 );
+
+        BOOST_TEST( r.has_value() );
+        BOOST_TEST_EQ( *r, 2 );
+        BOOST_TEST_EQ( Y::instances, 0 );
+    }
+
     return boost::report_errors();
 }